Commercial aviation regulations govern many aviation activities associated with commercial aircraft transportation. Some of these regulations directly and indirectly place restrictions and limits on commercial aircraft brake design and operation by specifying the braking power capacity and margins necessary to meet safety and emergency conditions. Braking power capacity is the ability of aircraft brakes to absorb kinetic energy generated in slowing or stopping aircraft movement. Aircraft brakes absorb the kinetic energy by converting the kinetic energy of the aircraft into thermal energy by raising the thermal mass temperature of the brakes. The brakes in turn transfer this thermal energy to the surrounding environment through thermal heat transfer mechanisms, which include conduction, convection, and radiation. These heat transfer mechanisms are dependent on mass, temperature and time. The thermal mass temperature of the brakes at any given time is a measure of the thermal energy stored in the brakes at the given time. The capacity of the brakes to absorb additional kinetic energy is determined by the difference in the thermal mass temperature at the given time subtracted from a predetermined maximum thermal mass temperature of the brakes. Therefore, the frequency and magnitude of kinetic energy absorbed by the brakes is dependent on the time it takes for the brakes to cool. Based on the foregoing, the thermal mass temperature of the brakes prior to a braking action determines the thermal mass temperature of the brakes at the conclusion of the braking action. For this reason, the brake cooling rate directly affects the utilization of the braking capacity of the brakes.
In addition to rules and regulations pertaining to braking power capacity, there are other aircraft configuration and system requirements that drive, and are driven by, the brake temperature, such as wheel fuse plug melt temperature and wheel well material temperature limits. These aircraft configuration and system requirements, as well as the rules and regulations, ensure that the temperature of an aircraft brake, and its associated components, remains below a maximum allowable temperature threshold during certain operating conditions or flight schedules. For example, the temperature of the brakes of an aircraft cannot exceed a threshold temperature during or following a preset number (e.g., four) of short distance flights, which are associated with relatively rapid takeoffs and landings. According to another regulation, the temperature of the brakes of commercial aircraft must be below some other threshold temperature before the aircraft is allowed to depart from a gate for takeoff.
Such rules, regulations, and requirements present challenges for some aircraft and brake types, as well as achieving certain flight schedules. The brakes of an aircraft generate a substantial amount of heat via the absorption of kinetic energy associated with slowing down an aircraft upon landing. For example, aircraft brakes on certain commercial aircraft may reach extremely high temperatures (e.g., 900° F.-1,100° F.) during landing. As soon as an aircraft has slowed down to a taxiing speed after landing, the brakes immediately begin to cool. However, the rate of cool down can be slow and often is inhibited by frequent braking during the taxi phase, which may raise the temperature of the brakes above the original landing temperature. While stationary at the gate, the cool down rate of the brakes typically is extremely low. Natural convective cooling may also take place during takeoffs and flight. However, like the convective heat transfer from the brakes during taxiing and at the gate, the convective heat transfer rate from the brakes during takeoff and flight is relatively low. In view of the high temperatures reached to slow the aircraft down during landing, and the relatively slow rate of heat transfer by natural convective cooling on the ground or in flight, it may be difficult to meet the maximum allowed brake temperature without undesired consequences in some cases. For example, an aircraft may not be in the air or on the ground long enough between landings to meet the repetitive-short flight regulation. Likewise, the gate departure regulation often results in aircraft departure delays as personnel must wait until the temperature of the brakes drop below the regulated threshold, which may cause airlines to miss desired gate turnaround times and associated flight frequency quotas.
Desirably, to meet the brake temperature limits and regulations while avoiding undesired consequences, some aircraft and aircraft component manufacturers, as well as operators, have recognized the need to improve cooling of the brakes. However, conventional methods and techniques employed to improve cooling often fail to adequately cool the brakes fast enough to either meet the brake temperature regulations or desired objectives, and often require the addition of ancillary power systems and other components.
For example, operators may remove a seal between a landing gear door and the body of an aircraft to allow external air to passively flow into the landing gear cavity during flight. Although the flow of air through the landing gear cavity may increase the convective cooling of the brakes, the rate of heat transfer still may not be high enough to sufficiently cool the brakes.
Other techniques involve the use of an electrically-powered fan that generates an artificial flow of air across the brakes during flight. Notwithstanding the ability of such active cooling techniques to lower the temperature of the brakes, these active temperature control techniques require additional electrical systems and power consumption in order to reduce brake temperatures. Not only do active cooling systems introduce reliability concerns commonly associated with electrical components and controls, but active cooling systems require and consume large amounts of power for operation, which can lead to substantial costs over time.
Some aircraft manufacturers and operators use techniques to reduce the temperature of the brakes reached during landing to obviate the need for auxiliary brake cooling. For example, increasing the mechanical braking capacity of the brakes tends to reduce maximum temperature reached during landing. However, increasing the mechanical braking capacity of the brakes also brings some undesired consequences, such as increased weight, cost, and complexity. Operators have also employed thrust reversing techniques or increased existing reverser settings on landing to decrease the maximum temperature of the brakes reached during landing. Although such techniques may result in a decrease in the brake heat loads (e.g., maximum temperature of the brakes), the decrease may not be sufficient to meet brake temperature regulations and/or avoid undesired consequences, such as the inability to meet faster, more frequent flight schedules, as well as placing additional stresses on propulsion systems and aircraft structures. In some instances, thrust reversing degrades the stopping capability of the aircraft by unweighting the landing gear, which results in increased stopping distance and places an even greater burden on the braking capacity.
In addition to concerns associated with meeting commercial aviation regulations, high aircraft brake temperatures may also cause undesirable temperature increases in components near the aircraft brakes when the brakes are stored after takeoff. Some aircraft materials, such as composite materials, may not tolerate extreme temperature increases due to radiated heat transfer from the stored brakes to the materials. To reduce the temperature increase of components near the brakes, some aircraft manufacturers install an insulation layer between the brakes and the components. However, insulation layers tend to increase the cost of the aircraft and reduce space within the aircraft.